Watch bracelets or watchbands are either of the open type or of the closed type. Understood by open-type is one whose two ends can be disunited. The ring formed by the watch and the bracelet can thus be opened to slip on the bracelet. Understood by closed-type is one whose two ends remain integral. Bracelets of the closed type generally comprise an unfolding clasp enabling the diameter to be increased of the ring formed by the bracelet and the watch, so that the hand can pass through. The present invention concerns just such an unfolding clasp for a bracelet or band, in particular one of the closed type.
Different types of unfolding clasps are known, all of which however generally comprise at least two mutually articulated plates as well as a closure. In is closed position, the plates are generally folded up, one on the other, or one next to the other, and are held in this position by an appropriate device. By actuating the closure, the user can release this device to unfold the articulated plates in such a way as to enlarge the opening of the bracelet. The patent documents FR 2 571 228, FR 2 577 120, CH 684 151, CH 678 002 and CH 635 237, for example, describe different types of unfolding clasps for bracelets or bands.
The unfolding of the clasp must be sufficient to allow, in opened position, passage of the hand through the opening of the bracelet, while sufficiently clasping the wrist when in closed position. The common unfolding clasps generally allow the circumference of the bracelet to be varied by about three to six centimeters.
Different constructions have been conceived, with a variable number of plates folding up in diverse ways, which allow such an unfolding to be obtained. The width and the thickness of the plates, however, must be sufficient to ensure reliable functioning of the unfolding clasp, even after a large number of manipulations. The number of articulations is limited, particularly by cost constraints. Moreover, generally desired is that the width of the closure in the closed position allows the closure to be hidden under the bracelet, whereas its thickness must be reduced for reasons of comfort and aesthetics. For all these reasons, the number of usable constructions is limited in actual fact, and all involve relatively long plates. For example, if the clasp is formed by two plates folding up one on the other, the length of the plates in closed position must be at least five centimeters to allow a spreading out of five centimeters in opened position. With three plates folding up in accordion fashion, one on the other, the length of the plates and the clasp in closed position is more than 2.5 centimeters, for example. It is difficult to go beyond three superimposed plates without increasing the thickness of the closure in an unacceptable way.
The length of the unfolding clasp in closed position thus has to be rather large. Consequently, the plates forming the clasp are generally curved to correspond approximately to the curvature of the wrist of the wearer of the watch. The adopted curvature must necessarily be a compromise corresponding to a common wrist size, and thus does not fit very thin wrists or the contrary, wrists wider than average. For this reason, many people are of the opinion that bracelets or bands with unfolding clasps are not very comfortable, and nevertheless prefer the less practical bracelets or bands of the open type.
Telescopic unfolding clasps or those of variable length have been proposed, for example, in CH 668 353 and EP 0 453 635. These devices are however of complex construction and fragile.